We describe the macroalgal assemblages in Potter Cove and their distribution by depth, substratum, irradiation, nutrients, salinity, water temperature and light period. We determined groups by using K-means, the Hellinger index and the maximum Calinski-Harabasz pseudo-F-statistic as stopping rule. Indicator species were assessed with the IndVal index. Relationships between groups and environmental factors were determined by correspondence analysis, and results were validated by a canonical correspondence analysis of species and environment. To compare the group biodiversities we used species richness and diversity indices and abundance biomass comparisons curves. Analysis with incidence data discriminated one shallow Iridaea assemblage and one deep Himantothallus assemblage. Analysis using biomass discriminated three facies, the first in the upper zone (Iridaea cordata, Monostroma hariotii, Adenocystis utricularis, Enteromorpha bulbosa), the second (Desmarestia menziesii, D. antarctica, Ascoseira mirabilis, Curdiea racovitzae, Gigartina skottsbergii, Kallymenia antarctica, Neuroglossum delesseriae, Phaeurus antarcticus, Gymnogongrus turquetii) at intermediate depth and the third (H. grandifolius, D. anceps, Georgiella confluens, Ballia callitricha, Plocamium cartilagineum), coincident with the deep qualitative assemblage, was found in a less disturbed environment. The difference in species composition between the groups was mainly related to depth as an environmental variable. The high specific diversity at intermediate depths can be explained by the presence of patches in different successional stages caused by iceberg scouring.

The biomass variation and the reproduction of the natural Gracilaria gracilis bed in Bahía Bustamante (Patagonia, Argentina) were analyzed for 2 years, with the aim of determining the present situation of the population for an updated status overview; establishing the relevant features of the temporal variation in both biomass and reproductive states in relation to environmental factors, epiphytes and associated algae; and assessing carpospore availability for future spore-culture development. Field measurements and sampling were performed monthly between March 2006 and February 2008. In both years, G. gracilis biomass presented marked seasonal variations, with a minimum in winter and a maximum in late spring and in summer. During both years, coexistence of the three life-cycle phases was found, with dominance of tetrasporophytes. Two data sets from individuals originated from sexual reproduction (tetraspores and carpospores) and from asexual reproduction by thallus fragmentation were analyzed separately. In the fragmentation fraction, tetrasporophyte frequencies remained higher than those for gametophytes. However, in the spore-originated fraction, a generation ratio close to 0.5 was observed. Female gametophytes bearing cystocarps were always present, with a maximum in summer and autumn. Biological data were related to environmental factors by means of canonical correspondence analysis (CCA). The first year was characterized by higher biomass values of G. gracilis and Undaria pinnatifida, lower epiphytism, larger Gracilaria thalli and greater proportion of mature tetrasporophytes and gametophytes. The second year was characterized by a high proportion of Gracilaria vegetative thalli and high epiphyte density. The best time to obtain spores from cystocarpic thalli would be in summer and early autumn.

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